Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $12.28   6.449%   $13.0719   $13.07   $13.05   $13.10  
  Purchase #2 $122.83   12.177%   $137.787   $137.79   $137.80   $137.80  
  Purchase #3 $236.78   11.220%   $263.3467   $263.35   $263.35   $263.30  
  Purchase #4 $109.70   12.212%   $123.0966   $123.10   $123.10   $123.10  
  Purchase #5 $198.44   11.275%   $220.8141   $220.81   $220.80   $220.80  
  Purchase #6 $194.63   3.656%   $201.7457   $201.75   $201.75   $201.70  
  Purchase #7 $67.80   5.402%   $71.4626   $71.46   $71.45   $71.50  
  7 purchase totals = $942.46       $1,031.3246   $1,031.33   $1,031.30   $1,031.30